The development of autoimmunity involves the failure of the mechanisms that regulate the ability to discriminate self from non-self. The primary means of regulating self-tolerance is through the deletion of self-reactive cells in the thymus. However, this mechanism is not perfect and auto-reactive clones do escape into the periphery. Peripheral tolerance is generated through a variety of mechanisms including T cell anergy and T cell indifference or ignorance. Recently, another, more active, mechanism of tolerance induction has been identified that is controlled by a population of regulatory T cells that actively suppress the function of auto-reactive T cells. These T cells, known as T/R cells, have a characteristic cell-surface phenotype and fail to respond to typical T cell stimulation regimens in vitro. However, they have the ability to inhibit the development of autoimmunity when transferred into the appropriate host. However, the mechanism by which these cells perform their tasks is as yet unclear. Recent work has shown that the forkhead/winged-helix protein FoxP3 is expressed predominantly in TR cells, and is both necessary and sufficient for their development and function. FoxP3 acts as a transcriptional repressor, targeting cytokine genes whose expression is induced in stimulated CD4+ T cells. However, the mechanism underlying the ability of FoxP3 to drive the development of TR cells remains unclear. Another question that is unanswered is whether FoxP3 expression is required continuously for a T cell to be a T/R cell, or whether expression is required only at specific times during T cell development and maturation. The experiments in this application address this question through the use of mice that express inducible FoxP3 transgenes. In this way, FoxP3 expression can be manipulated under conditions designed to assess T/R cell function. The experiments will also begin to test the feasibility of manipulating FoxP3 expression as a means of treating autoimmune disease.